To achieve high detection sensitivity in a mass spectrometry device, it is important for ions derived from sample components generated in an ion source to be fed into the mass spectrometer, such as a quadrupole mass filter, etc, as efficiently as possible. In particular, in mass spectrometry devices such as liquid chromatography-mass spectrometry device, where ionization is performed under atmospheric pressure, even under conditions of low vacuum atmosphere, i.e. when there are relatively many residual gas molecules, it is important to reduce the influence of scattering due to collision with such gas molecules as much as possible, and to transport ions to the mass spectrometer while minimizing losses. To achieve this objective, an ion optical element known as an ion guide is used for focusing the ions sent from the preceding stage and feeding them into the mass spectrometer, etc. of the next stage.
The general configuration of an ion guide is a multipole configuration in which 4, 6, 8 or more substantially round cylindrical rod electrodes are spaced apart from each other at the same angle and arranged in parallel to each other so as to surround the ion optical axis. In a multipole ion guide of this sort, normally, high frequency voltages of the same amplitude and frequency but of inverted phase are applied respectively to two rod electrodes adjacent in the circumferential direction about the ion optical axis. When this sort of high frequency voltage is applied to each rod electrode, pseudo-potential barriers are formed by the high frequency electric field generated between the electrodes, and ions are reflected between these potential barriers as they travel downstream. As a result, ions scattered due to collision with residual gas molecules can also be stably transported and the sensitivity of the device can be increased.
Quadrupole, hexapole and octupole configurations are commonly used for multipole ion guides. It is known that when the voltage applied to the rod electrodes is the same, the greater the number of poles, the greater the ion confinement potential in the vicinity of the rod electrodes. It is furthermore known that the ability to focus ions near the ion optical axis is higher when the number of poles is smaller. FIG. 8 is a drawing which schematically illustrates the relationship between radial distance r from the ion optical axis (center) and the confinement potential φ in a quadrupole ion guide and an octupole ion guide (see Patent literature 1, etc.).
It can be seen that in an octupole ion guide, the confinement potential rises sharply and the ion confinement capacity is higher at locations near the rod electrodes (away from the center). On the other hand, since the bottom of the potential well is wide, ions can be readily present not just near the ion optical axis but also at locations away from the optical axis. In other words, the degree of concentration of ions toward the vicinity of the ion optical axis is not particularly good. By contrast, with a quadrupole ion guide, the confinement potential rise is gradual, so the ion confinement capacity is relatively low, but the bottom of the potential well is limited to a narrow range in the vicinity of the ion optical axis, so ions are focused near the ion optical axis.
It will be noted that in a quadrupole ion guide, the confinement potential can be increased by increasing the amplitude of the high frequency voltage applied to each rod electrode, but a quadrupole ion guide has a low mass cutoff (LMC) limiting condition (see Patent literature 2, etc.), with the LMC increasing the more one raises the driving voltage. Thus, when driving voltage is raised in order to increase the confinement potential, the problem occurs that it becomes difficult to stably transport ions with a low mass-charge ratio, so there are limits to increasing the driving voltage.
Since the ion transport characteristics differ in this way between quadrupole ion guides and octupole ion guides, and also multipole ion guides with other numbers of poles, it is desirable to select an ion guides with the appropriate number of poles according to the conditions of use, such as the mass-charge ratio range of the ions to be analyzed. Specifically, when analyzing ions across a wide mass-charge ratio range, it is preferable to use to an octupole ion guide with high confinement capacity, and to detect ions with a specific mass-charge ratio or ions with a narrow mass-charge ratio range at high sensitivity, it is preferable to use a quadrupole ion guide, focus ions near the ion optical path and transport ions to the subsequent stage ion optical system at low loss. Because of this, in order to obtain good analysis results, it is desirable to be able to rapidly switch the effective number of poles of the multipole ion guide even during execution of liquid chromatography/mass spectrometry (LC/MS) or gas chromatography/mass spectrometry (GC/MS).
However, in conventional mass spectrometry devices, switching the effective number of poles as described above is difficult for the following reasons. Namely, the high frequency voltage applied to each rod electrode of the multipole ion guide requires an amplitude of approximately several hundred V, and to generate such a voltage, LC resonant circuits employing inductance and capacitance are generally used in the prior art. FIG. 7 is a simplified diagram showing the electrode configuration and driving circuit of a conventional octupole ion guide.
In FIG. 7, the eight rod electrodes 21 through 28 contained in ion guide electrode unit 2 are arranged so as to be inscribed into a virtual round cylindrical body P having the ion optical axis C at its center and so as to be spaced apart at equal angular intervals (45°) in the circumferential direction. Sets of four of these eight rod electrodes 21 through 28, consisting of every other one in the circumferential direction (rod electrodes 21, 23, 25 and 27; and rod electrodes 22, 24, 26 and 28) are electrically connected, and voltage from a power supply unit 500 is applied to each of these two electrode groups. Looking at the ion guide electrode unit 2 from the power supply unit 500, an electrostatic capacitance C′ exists between circumferentially adjacent rod electrodes, and this electrostatic capacitance C′ is connected in parallel to a variable capacitance capacitor 503 having a capacitance C. The LC resonant circuit, formed by this electrostatic capacitance C′ and capacitance C of variable capacitance capacitor 503 and the inductance L of coil 502, increases the amplitude of the high frequency signal inputted from high frequency signal generating unit 501, which is then applied to the rod electrodes 21 through 28. The resonant frequency is fixed, and the capacitance C of the variable capacitance capacitor 503 is adjusted to match the resonant frequency fLC of the LC resonant circuit to a specific frequency f.
In FIG. 7, if four electrode pair sets are formed taking two circumferentially adjacent rod electrodes as one set, and the electrical connection is switched by a switching means such as an electromagnetic relay so that a high frequency voltage of reverse polarity is applied to circumferentially adjacent electrode pairs, a quadrupole electric field can be formed in the space surrounded by rod electrodes 21 through 28. That is, the effective number of poles can be switched from 8 to 4. However, when this sort of switching is performed, the electrostatic capacitance C′ between the rod electrodes changes, and thus the resonant frequency fLC of the LC resonant circuit deviates from the specific frequency f and adequate amplification of amplitude becomes impossible. In other words, high speed switching as described above was not possible because the capacitance C of variable capacitance capacitor 503 needs to be readjusted in response to change in electrostatic capacitance C′ between the rod electrodes in order to modify the effective number of poles. Furthermore, the switching itself was a very laborious operation and was not practical.